The bacterial replisome: back on track?

Abstract

Summary It has been postulated that bacterial DNA replication occurs via a factory mechanism in which unreplicated DNA is spooled into a centrally located replisome and newly synthesized DNA is discharged towards opposite cell poles. Although there is considerable support for this view, it does not fit with many key observations. I review new findings, and provide alternative interpretations for old findings, which challenge this model. As a whole, current data suggest that the replisome, at least in slowly growing Escherichia coli cells, tracks along a stationary chromosome. These replisomes are not stationary, tethered or restricted in their movement, but rather travel throughout the nucleoid. One possibility is that the replisome navigates along a chromosome made up of looped domains as has been previously envisioned.

Fig. 1. Models for DNA replication

A. Tracking replisomes. Two independent replisomes (green) track along a rosette-shaped parental chromosome (black lines), leaving two nascent daughter duplexes (red lines). B. Spooling replisomes. Replisomes remain within a central replication zone (hatched region), as parental DNA is threaded through replication zone (black arrows) and daughter duplexes are ejected outward (red arrows).

Fig. 2. Two configurations for spooling replisomes

A. Tail-to-tail replisomes. Helicases (green) are oriented in opposite directions. B. Head-to-head replisomes. Helicases are positioned near each other. Pol III core (red) and B clamp (brown) are shown. Lagging strand is denoted by looped portion. Arrows indicate motion of DNA (grey) relative to fixed replisomes. Drawing is not to scale, and is highly schematic. See text for details.